Many people like to ask whether the future is determined. This has relevance to issues like free will, for example.
I would like to address the reverse question: Is the *past* determined?
In other words, is the past fixed once we pass it?
Another interpretation of the problem, possibly a different spin: Given the state of the universe *now* (and I am flexible about what this means), is the entirety of the past determined? Can every event of the past be *theoretically* deduced from the information of the present?
When framed in this way, I have to say that it seems unlikely. For example, it seems quite unlikely that the weight of Casar's last drink is fixed from anything available in the universe today. It seems unlikely that the question 'was there a T-Rex standing in this spot 68 million years ago exactly' actually has an answer that is determined by the state of the universe now.
So, to what extent is the past determined? If it is NOT determined, how does that affect your views of the past? if it *is* determined, in what sense is it so?
Einstein regarded spacetime as something modern expounders can liken to a loaf of bread in 4-space, which relativity cuts into slices to illustrate what things like before, after, and simultaneous mean if general relativity is correct. It's not hard to interpret that as a claim that the past (and, relatively speaking) bits of the future are fixed.
And as you know, the present scientific approach to reasoning from retrospect takes the past to be fixed, and to be consistent in its obedience to the rules. Thus we make models from hypotheses about past states and see how well their results match the present when cranked through the model's prescribed processes. Examples are models of the universe from the Big Bang to the present distribution of galaxies &c, and models of the formation and evolution of the solar system. I have no doubt there are a great many more.
Do I agree that the past is fixed? Yes, I think our past is the past and I think the past as such can't be changed after it's happened. Certainly I see no evidence in the present that humans from the future are among us ─ and maybe if time travel were indeed possible, there'd be very good reasons not to do it.
But was the past always fixed? I recall the late Stephen Jay Gould's
Wonderful Life and his argument that if you "played the tape of life again" starting at the Burgess Shales, you'd get a totally different set of animals to the ones we have, such are the sheer chances at large in evolution. This is underlined by the mechanics of human conception, where the genetic composition of the offspring depends on which specific spermatozoon forms the zygote with that particular ovum. A few seconds earlier or later, or a different positioning of the bodies, or a different month, and you'd very likely get a different result.
None of that matters if determinism is strict at all levels; but since we don't know whether random quantum events can affect macro results on a scale which would disturb such outcomes, we don't presently have a firm answer to that question.
When we enlarge the scale to look at the formation of the universe, we find its dust, rocks, rings, planets, stars, galaxies, galaxy clusters and superclusters, its dark matter and dark energy and the likelihood of more unknown unknowns, all in particular spatial relationships with each other.
Are such large-scale arrangements so big that random quantum events don't make a significant difference? If we play the universe's tape again, will we get the same entities and the same relationships? I'm not aware that we have an answer to that either.
Which is to say, although we have only one past, it may be possible ─ but we don't know how possible ─ that if we ran
that tape again, we'd get a different set of relationships of the elements of the universe in both senses of the word.